Why is 17-40 not suitable for taking portraits?

The so-called "art has laws, art can't."

Generally, the middle corner is suitable for taking portraits, because its perspective is not exaggerated (it can avoid over-emphasizing the nose of the character), there is no distortion, and the distance is appropriate when taking portraits, which is convenient for establishing good emotional communication and spiritual contact with the subject, and is conducive to expressing the figure and facial features, facial expressions and skin texture of the character, and depicting the inner feelings and personality of the character.

But it doesn't mean that other focal lengths can't shoot portraits.

Generally, a lens with a focal length of F2.8-200 for wedding photography requires a professional zoom lens with a larger aperture.